Liquid crystalline 4&#34;-cyano- or 4&#34;-nitrobenzylidene-4&#39;-(N,N-dialkylamino)-1-aminoazabenzene dyes

ABSTRACT

Compounds of the formula ##STR1## wherein Y and Z are different and one is nitro or cyano and the other is --N(R) 2  wherein R is alkyl or alkylene, preferably of 1 to 4 carbon atoms, and X can be hydrogen, methyl, fluoro or chloro, are dyes having liquid crystal properties.

This invention relates to novel liquid crystal devices. Moreparticularly, this invention relates to liquid crystal mixtures anddevices containing novel liquid crystalline dyes.

BACKGROUND OF THE INVENTION

Electro-optic devices containing liquid crystal compounds have becomecommercially important recently because of their low power requirementsand good contrast, particularly for applications such as watch faces,calculator displays and the like. Although liquid crystal compounds havebeen known for many years, the discovery of nematic liquid crystalshaving a mesomorphic transition temperature range that spans roomtemperature sparked a renewed interest in these materials and greatlyexpanded their marketability. Research is continuing to discover newroom temperature liquid crystal compounds and mixtures of liquid crystalcompounds having improved contrast in electro-optic devices.

SUMMARY OF THE INVENTION

We have discovered novel liquid crystal dye compounds, 4"-cyano- or4"-nitro-benzylidene-4'-(N,N-dialkylamino)-1-aminoazobenzenes and4"-N,N-dialkylaminobenzylidene-4'-cyano- or 4'-nitro-1-aminoazobenzene.These liquid crystals, when admixed in small amounts with low meltingliquid crystal materials, impart strong colors to the mixtures, butwithout materially affecting the transition temperature range of theliquid crystal composition to which they are added. The present dyesthus improve the contrast of electro-optic devices employing theseliquid crystal materials, and impart decorative colors in the orange tored to brown hues, without adversely affecting other properties of theliquid crystals.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a cross-sectional view of an electro-optic deviceembodying the invention.

DETAILED DESCRIPTION OF THE INVENTION

The novel liquid crystal dyes of the present invention have the formula:##STR2## wherein Y and Z are different and one is nitro or cyano and theother is --N(R)₂ wherein R is alkyl or alkylene of 1-8, preferably of 1to 4 carbon atoms, and X can be hydrogen, methyl, fluoro or chloro.

These dyes encompass a rather broad spectrum of mesomorphic temperatureranges and have deep colors. They can be admixed with lower meltingliquid crystal compositions in small amounts to impart color to theliquid crystals so that cells including them exhibit improved contrast.

The above-described liquid crystalline dyes can be prepared by reactingan appropriate benzaldehyde and a suitable aminoazobenzene orsubstituted derivative thereof in solution. Para-toluenesulfonic acidcan be employed as a catalyst. The product is isolated and purified inknown manner, as by recrystallization or column chromatographictechniques.

The dyes can be admixed with nematic liquid crystal compositions toimpart a color characteristic of each dye. Low melting nematic liquidcrystal compositions, particularly p-alkoxybenzylidene-p'-alkylanilinesand their mixtures with other liquid crystals, and mixture of p-alkoxy-or p-acyloxybenzylidene-p'-cyanoanilines, or mixtures ofp-cyanophenyl-p'-alkylbenzoates and p-phenylene-2,4-dialkylbenzoateshave a transition temperature range that includes room temperature, andare particularly suitable. The exact amount of dye to be added dependson the solubility of the dye in the liquid crystal composition and alsoon the color desired. In general, from about 0.05% up to about 2% byweight of the dyestuff in the liquid crystal mixture, and preferablyfrom about 0.2% to 0.4% by weight of the dyestuff, will be employed.

P-alkoxybenzylidene-p'-butylanilines are known liquid crystals and aredescribed for example in U.S. Pat. No. 3,829,491 which issued Aug. 13,1974 to Strebel. Mixtures of p-methoxybenzylidene-p'-n-butylaniline(hereinafter referred to as MBBA) andp-ethoxybenzylidene-p'-n-butylaniline (hereinafter referred to as EBBA)have particularly broad and low use temperature ranges. Mixturescontaining about 35 to about 70% by weight of MBBA are preferred.

P-alkoxybenzylidene-p'-cyanoanilines are also known and are described inU.S. Pat. No. 3,499,702issued Mar. 10, 1970, to Goldmacher et al. Amixture containing 85% by weight of a 70:30 molar ratio of MBBA and EBBAand 15% by weight of p-ethoxybenzylidene-p'-aminobenzonitrile has beendisclosed by H. A. Tarry, Services Electron Research LaboratoryTechnical Journal, Vol. 23, No. 1, 1973.P-acyloxybenzylidene-p'-cyanoanilines have been disclosed by Castellanoin U.S. Pat. No. 3,597,044.

In preparing an electro-optic device, the liquid crystal compoundsshould be rigorously purified to remove ionic and nonionic impuritieswhich may react to degrade the liquid crystal compounds either bydecomposition, transubstitution reactions and the like. For commerciallyacceptable cells, the liquid crystal compounds should be purified sothat their resistivity is 1 × 10¹¹ ohm-cm or higher.

After the desired liquid crystal compounds are mixed together, a smallamount of a chiral aligning agent is advantageously added. The aligningagent serves to orient the molecules of the mixture in the samedirection since chiral compounds normally twist in a right-handed orleft-handed direction. Suitable aligning agents include cholesterylderivatives, such as cholesteryl halides, cholesteryl esters and thelike; optically active compounds such as d- or 1-α-pinene, d- or1-octanol and chiral esters such as4-propylphenyl-4'-(4"-2-methylbutylphenylcarboxy)-2-chlorobenzoatehaving the formula ##STR3## and the like. The amount of the chiralaligning agent added is not critical, but in general from about 0.05 toabout 2.0% by weight of the liquid crystal composition is suitable,providing the mixture remains nematic.

Referring to the FIGURE, a liquid crystal cell is constructed from twoglass plates 2 and 4 having conductive indium-doped tin oxide coatingsas electrodes 6 and 8, respectively, on facing surfaces thereof. Thinsilicon oxide layers 10 and 12 are evaporated onto the conductive layers6 and 8, respectively, at an angle of 60°. These layers act to align theliquid crystal material. One-half mil (25.4 microns) thick glass fritspacers 14 and 16 maintain the coated glass plates 2 and 4 apart tocomplete the cell components. The cell is baked at 525° C. to melt theglass frit and seal the cell except for a single fill port. The cell isfilled with the desired liquid crystal composition 18 in the isotropicstate and hermetically sealed with a solder.

The invention will be further illustrated by the following Examples butit is to be understood that the invention is not meant to be limited tothe details disclosed therein. In the Examples, parts and percentagesare by weight unless otherwise noted.

EXAMPLE 1 Preparation of4"-cyanobenzylidene-4'-(N,N-din-hexylamino)-3-methyl-1-aminoazobenzene

A reaction mixture of 0.53 gram (3.5 millimols) of p-cyanobenzaldehyde,0.79 gram (2 millimols) of4'-(N,N-di-n-hexylamino)-3-methyl-1-aminoazobenzene, 150 ml of benzeneand about 3 mg of p-toluenesulfonic acid was charged to a reactionvessel equipped with a magnetic stirrer and a reflux condenser toppedwith a graduated cylinder Claison adapter. The mixture was stirred atreflux until reaction had stopped as monitored by thin layerchromatography. The reaction mixture was filtered through a 150 gramsilica gel bed and the product eluted with benzene. The solvent wasremoved by a flash rotary evaporator and the product purified byrepeated column chromatography.

The product was recrystallized from hexane repeatedly until a thin layerchromotogram showed only a single spot.

4"-cyanobenzylidene-4'-(N,N-di-n-hexylamino)-3-methyl-1-aminoazobenzenehaving the formula ##STR4## was obtained in 72% yield (0.73 gram).

The structure was confirmed by elemental analysis: C calculated, 78.11%;Found, 78.29%; H calculated, 8.09%; Found, 8.14%.

This compound was orange-red in color and was monotropic, having a solidto nematic transition temperature of 94-95° C. and a melting point of110.5° C.

EXAMPLES 2 - 11

Several additional compounds were prepared. The general procedure ofExample 1 was followed, substituting the appropriate starting materials.Table I below summarizes the analyses, mesomorphic temperature rangesand colors of the dyestuffs obtained.

                                      Table I                                     __________________________________________________________________________                                    Analytical Data                                                               Car-                                                                              Car-                                                                      bon,                                                                              bon,                                                                              H,             Mesomorphic                                            calc,                                                                             found                                                                             calc,                                                                            H,  Yield   Temperature            Ex.                                                                              Compound                     %   %   %  found                                                                             %   Color                                                                             CN,°                                                                       NL,°        __________________________________________________________________________                                                               C                   2                                                                                ##STR5##                    74.79                                                                             74.76                                                                             5.38                                                                             5.43                                                                              62  orange- red                                                                       248 252                 3                                                                                ##STR6##                    75.59                                                                             76.04                                                                             6.04                                                                             6.15                                                                              57  orange- red                                                                       211 255                 4                                                                                ##STR7##                    75.94                                                                             75.63                                                                             6.33                                                                             6.43                                                                              60  red 151.3                                                                             103.3- 111.3*       5                                                                                ##STR8##                    68.83                                                                             68.72                                                                             5.74                                                                             5.57                                                                              78  red 235 257                 6                                                                                ##STR9##                    69.40                                                                             69.54                                                                             6.02                                                                             6.18                                                                              64  red- brown                                                                        164 120- 132*           7                                                                                ##STR10##                   75.99                                                                             75.60                                                                             5.54                                                                             5.55                                                                              33  red 187 236                 8                                                                                ##STR11##                   76.89                                                                             76.59                                                                             7.09                                                                             7.16                                                                              37  orange- red                                                                       72  175                 9                                                                                ##STR12##                   77.16                                                                             77.54                                                                             7.32                                                                             7.46                                                                              68  orange- red                                                                       54  122                10                                                                                ##STR13##                   70.90                                                                             70.70                                                                             6.78                                                                             6.77                                                                              38  orange- red                                                                       84  184                11                                                                                ##STR14##                   77.89                                                                             78.02                                                                             7.91                                                                             7.84                                                                              51  orange- red                                                                       88.5                                                                              129                __________________________________________________________________________     *monotropic                                                              

EXAMPLE 12

The liquid crystal dyes prepared in Examples 1-11 were admixed withlower temperature mixtures of liquid crystals and the mesomorphictemperature ranges of the mixtures measured.

The Schiff base liquid crystal host contained 85% by weight of a 70:30mol ratio of MBBA and EBBA and 15% by weight ofp-ethoxybenzylidene-p'-aminobenzonitrile. This mixture has a nematic toisotropic liquid transition temperature of 63° C.

The ester liquid crystal host is available commercially fromHoffman-LaRoche as RO-TN-101 and is a mixture of three parts ofp-cyanophenyl-p'-alkylbenzoates of the formula ##STR15## and one part ofa p-phenylene-2,4-dialkylbenzoate of the formula ##STR16## wherein R',R" and R'" are alkyl groups.

The data are summmarized below in Table II.

                  Table II                                                        ______________________________________                                                                       Nematic to                                                                    Isotropic                                      Compound Host Liquid Weight %  Transition                                     Sample No.                                                                             Crystal     of Dye    Temperature, ° C                        ______________________________________                                        1        Ester       0.33      48-66                                          2        Ester       0.39      70.7-72.5                                      2        Schiff Base 0.228     31.5                                           3        Ester       0.34      70.7-71.5                                      3        Schiff Base 0.203     47                                             4        Ester       0.19      69-71.5                                        5        Ester       0.26      70-70.5                                        5        Schiff Base 0.287     52                                             6        Ester       0.23      70-71.2                                        7        Ester       0.26      70.5-72                                        8        Ester       0.4       62.5-65                                        8        Schiff Base 0.224     28.5                                           9        Ester       0.40      69.5-71                                        10       Ester       0.38      70-71                                          10       Schiff Base 0.204     29                                             11       Ester       0.20      69-71.5                                        ______________________________________                                    

EXAMPLE 13

A mixture of 0.21% of the liquid crystal dye of Example 11 in the esterhost liquid crystal mixture of Example 12 was charged to a liquidcrystal cell as in the FIGURE. The liquid crystal mixture was aligned ina direction parallel to the electrode of the cell. The optical density,as seen through plane polarized light parallel to the liquid crystalorientation, was measured at the visible absorption maximum (475 nm) asa function of applied voltage. The results are given below in Table III.

                  Table III                                                       ______________________________________                                        Voltage                                                                       (volts)        Optical Density                                                ______________________________________                                        0              0.466                                                          0.5            0.466                                                          1.0            0.410                                                          1.2            0.364                                                          1.5            0.322                                                          2              0.297                                                          3              0.265                                                          4              0.255                                                          5              0.251                                                          8              0.243                                                          10             0.241                                                          15             0.239                                                          20             0.237                                                          ______________________________________                                    

The results show that as the voltage is increased, the color fades.

EXAMPLE 14

A mixture of 0.34% of the dye of Example 3 in the ester host liquidcrystal mixture of Example 12 was charged to a liquid crystal cell andoptical density measured as in Example 13, except at 486 nm. The resultsare given below in Table IV.

                  Table IV                                                        ______________________________________                                        Voltage (volts)  Optical Density                                              ______________________________________                                        0                0.816                                                        1                0.684                                                        1.5              0.624                                                        2                0.47                                                         3                0.354                                                        4                0.329                                                        5                0.319                                                        8                0.301                                                        10               0.298                                                        15               0.289                                                        20               0.286                                                        ______________________________________                                    

EXAMPLE 15

A mixture of 0.19% of the dye of Example 4 in the ester host liquidcrystal mixture of Example 12 was charged to a liquid crystal cell andoptical density measured as in Example 13, except at 470 nm. The resultsare given below in Table V.

                  Table V                                                         ______________________________________                                        Voltage (volts)  Optical Density                                              ______________________________________                                        0                0.530                                                        0.5              0.528                                                        1                0.508                                                        1.2              0.393                                                        1.5              0.341                                                        2                0.318                                                        2.5              0.289                                                        3                0.279                                                        4                0.272                                                        5                0.268                                                        10               0.261                                                        15               0.258                                                        20               0.257                                                        ______________________________________                                    

What is claimed is:
 1. A compound having the formula: ##STR17## whereinY and Z are different, one is nitro or cyano and the other is --N(R)₂wherein R is alkyl or alkylene of 1 to 8 carbon atoms, and X ishydrogen, methyl, fluoro or chloro.
 2. A compound according to claim 1wherein R is an alkyl group of 1 to 4 carbon atoms.
 3. A compoundaccording to claim 1 wherein Y is cyano, X is methyl and R is butyl. 4.A compound according to claim 1 wherein Y is nitro, X is hydrogen and Ris butyl.
 5. A compound according to claim 1 wherein Y is cyano, X ishydrogen and R is butyl.